The invention relates to systems and methods for communications of a signal containing information, and more particularly to communications systems and methods for coding the signal to generate multiple representations thereof.
Communications of audio information play an important role in multimedia applications, and Internet applications such as a music-on-demand service, music preview for online compact disk (CD) purchases, etc. To efficiently utilize bandwidth to communicate audio information, a perceptual audio coding (PAC) technique has been developed. For details on the PAC technique, one may refer to U.S. Pat. No. 5,285,498 issued Feb. 8, 1994 to Johnston; and U.S. Pat. No. 5,040,217 issued Aug. 13, 1991 to Brandenburg et al., both of which are hereby incorporated by reference. In accordance with such a PAC technique, each of a succession of time domain blocks of an audio signal representing audio information is coded in the frequency domain. Specifically, the frequency domain representation of each block is divided into coder bands, each of which is individually coded, based on psycho-acoustic criteria, in such a way that the audio information is significantly compressed, thereby requiring a smaller number of bits to represent the audio information than would be the case if the audio information were represented in a more simplistic digital format, such as the PCM format.
For example, in providing the aforementioned music-on-demand service, a server connected to the Internet may store PAC compressed versions of each available musical piece to serve client needs. Each version of the musical piece corresponds to a different connection speed at which a client, e.g., a personal computer (PC) having a modem, can afford to communicate over the Internet. The quality, or the lack of distortion, of the version of the musical piece increases with the connection speed corresponding thereto. Thus, for instance, if the server supports (a) a plain old telephone service (POTS) connection speed of about 28.8 kb/sec, (b) an integrated services digital network (ISDN) connection speed of about 64 kb/sec, and (c) a dual ISDN connection speed on the order of 100 kb/sec, three corresponding versions of the musical piece having the respective qualities need to be stored in the server. However, the storage of musical pieces in this manner is undesirably inefficient and occupies much memory space especially when a large number of musical pieces need to be made available.
In delivering the service to a client at a given connection speed, the server may packetize the corresponding audio information in the storage, and communicate the resulting packets through a packet switched network, e.g., the Internet. However, in the event that some of the packets are lost in transit because of imperfect network or channel conditions, which is likely, the quality of the received signal representative of a musical piece would be significantly degraded.
Accordingly, there exists needs for efficiently storing and distributing information at different rates, and effectively maintaining the minimum acceptable quality of the received signal despite imperfect network or channel conditions.
In accordance with the invention, multi-rate coding is implemented to generate multiple subrate representations of a signal containing information, e.g., audio information. These representations are different from one another and may be delivered at rates lower than or equal to the required delivery rates of the information.
For example, in providing the music-on-demand service described above, at least one of the subrate representations, referred to as a xe2x80x9cC-representation,xe2x80x9d may contain core information delivered at a subrate of 28.8 kb/sec. The other subrate representations, referred to as a xe2x80x9cE1-representationxe2x80x9d and xe2x80x9cE2-representation, each may contain enhancement information delivered at a subrate of 36 kb/sec. Because of the design of the multi-rate coding in accordance with the invention, in this instance recovery of the signal based on the C-representation alone affords the minimum acceptable 28.8 kb/sec signal quality; recovery of the signal based on the C-representation in combination with either E1-representation or E2-representation affords a higher 64 kb/sec signal quality; and recovery of the signal based on the C-representation in combination with both E1-representation and E2-representation affords the highest 100 kb/sec signal quality. Advantageously, the server of the aforementioned music-on-demand service needs to store in its memory the subrate representations, i.e., 28.8 kb/sec C-representation, 36 kb/sec E1-representation and 36 kb/sec E2-representation, of each musical piece, in lieu of the 28.8 kb/sec, 64 kb/sec and 100 kb/sec versions thereof as in prior art, to accommodate the different connection speeds and quality requirements, thereby effectively saving the memory space.
In accordance with an aspect of the invention, when the subrate representations are communicated to a client terminal in the form of packets, each packet includes at least an information content derived from one of the representations, and an indicator identifying the representation from which the information content is derived. Despite losses of some packets in transit because of imperfect channel or network conditions, the signal is recovered based on at least the received packets indicated to contain C-representation information to maintain the minimum acceptable signal quality.